The present invention relates to a control method and device for steering the rear wheels of a vehicle, whereby the rear wheels are steered in accordance with the steering of the front wheels.
According to this type of rear-wheel steering control method, when a vehicle turns, the steering amount for the rear wheels, which matches the steering amount of the front wheels and the car speed, is determined so that the rear wheels are steered in the same phase as the front wheels according to the determined rear-wheel steering amount. Steering the rear wheels in the same phase in this manner makes it possible to improve the posture of the vehicle, i.e., the traveling stability at the time of turning.
In other words, when the rear wheels are steered together with the front wheels, the gripping power of the tires increases, thus leading to improved traveling stability of the vehicle.
The conventional common rear-wheel steering control method is, however, based on an assumption that a vehicle travels on a traveling road surface which is flat and whose road surface friction coefficient (road surface .mu.) is large. This means that the rear-wheel steering amount is determined merely from the steering amount of the front wheels and the car speed.
Thus, in a case where a vehicle is traveling on a driving road with a smaller road surface .mu., what is called a "low-.mu. road," when the vehicle makes a turn, even if the rear wheels are steered in the same phase as the front wheels, the gripping power developed in the rear wheels will be insufficient because of the low road surface friction coefficient, resulting in deteriorated turning performance.
Also when a vehicle ascends or descends a slope, an appropriate steering amount of the rear wheels may not be obtained at the time of turning because of the longitudinal shifting of the load on the vehicle. To be more specific, when making a turn while descending a slope, the load on the rear wheels of the vehicle decreases, causing the gripping power of the rear wheels to decrease. This in turn causes the vehicle to develop rear-end shaking, and the turn of the vehicle shows an apparent oversteer tendency.
In contrast to the above, when the vehicle makes a turn while ascending a slope, the load on the rear wheels increases, and the gripping power of the rearwheel tires increases more than necessary. As a result, the rear end of the vehicle comes to easily turn, and the turn of the vehicle shows an apparent understeer tendency.
Accordingly, the turning performance becomes unreliable when the driving road surface has a low .mu. or when a vehicle ascends or descends a slope. The unreliability increases especially when traveling on a low-.mu. road or descending a slope.
As prior examples focused on the aforementioned point, technologies for detecting the friction coefficient of a road surface and correcting the rear-wheel steering angle in a direction along which the influences exerted by the road surface friction coefficient is reduced have been disclosed in Japanese Patent Applications Laid-Open S60-148769 (DE3500793, GB2153311, FR255813, US4964481).
Also in Japanese Patent Applications Laid-Open S62-8871 and H1-190585 have disclosed technologies for correcting the ratio of a rear-wheel steering angle to a front-wheel steering angle in accordance with the gradient of a road surface.
In these prior examples, however, corrections are made merely in accordance with the friction coefficient of a road surface or the gradient of a road surface; therefore these prior technologies presented a problem in that sufficient corrections cannot be made in response to road surface conditions that change while a vehicle travels, thus failing to assure stable effect of the rear-wheel steering.